In recent years, liquid crystal display devices have been broadly used as monitors of notebook-size portable computers and the like, or display sections of liquid crystal television sets or video integrated liquid crystal television sets, and further in other various fields. Each liquid crystal display device basically comprises: a backlight section; and a liquid crystal display element section. For the backlight section, an edge light system has been frequently used from a viewpoint of reduction in size of the liquid crystal display device. As a backlight, a system has heretofore been broadly used in which at least one end surface of a light guide having a rectangular plate shape is used as a light incident end surface, a linear or rod-shaped primary light source such as a straight tube type fluorescent lamp or the like is disposed along the light incident end surface, light emitted from the primary light source is introduced into the light guide via the light incident end surface of the light guide, and the light is output from a light outputting surface that is one of two main surfaces of the light guide.
This backlight has a problem that a sufficient quantity of light does not reach light guide corner portions in the vicinity of opposite end portions of the linear or rod-shaped primary light source, or regions of the light guide in the vicinity of side end surfaces of the light guide adjacent to the light incident end surface, and luminance easily drops in these portions or regions.
Additionally, in recent years, there has been a demand for miniaturization and reduction of power consumption with respect to a liquid crystal display device having a comparatively small screen dimension, for portable electronic apparatuses such as a cellular phone and a portable game machine, or indicators of various types of electric or electronic apparatuses. Therefore, a light emitting diode (LED) which is a point light source has been used as the primary light source of the backlight in order to reduce power consumption. In the backlight using the LED as the primary light source, a plurality of LEDs are one-dimensionally or linearly arranged along the light incident end surface of the light guide in order to exert a function similar to that of a backlight using a linear primary light source as described, for example, in JP(A)-7-270624. By the use of the primary light source by the one-dimensional arrangement of a plurality of LEDs in this manner, a desired quantity of light, and uniformity of a luminance distribution over the whole screen can be obtained.
However, in the small-sized liquid crystal display device, there has been a further demand for further reduction of the power consumption, and the number of LEDs for use needs to be reduced in order to meet the demand. However, when the number of LEDs is reduced, a distance between light emitting points lengthens, therefore a region of the light guide in the vicinity of the region between the adjacent light emitting points is enlarged, and intensity of light output in a desired direction from the light guide region drops. This brings about disproportionateness (i.e., brightness unevenness) of the luminance distribution of an observation direction in a light emitting surface of the surface light source device.
Moreover, in JP(B)-7-27137, a method has been proposed in which a light guide having a rough light outputting surface is used, a prism sheet having a large number of elongated prisms is disposed on the light outputting surface of the light guide in such a manner as to dispose a prism surface on a light guide side, and a distribution of the output light is narrowed in order to suppress the power consumption of the backlight and not to sacrifice the luminance as much as possible. In this backlight, although a high luminance is obtained with low power consumption, brightness unevenness is easily visually recognized through the prism sheet.
A most important problem in the brightness unevenness is a dark shadow part (dark part) generated in a light guide region corresponding to the outside of opposite-end LEDs 2 in the arrangement of a plurality of LEDs, or between the adjacent LEDs 2 as shown in FIG. 27. An actual generation example is shown in FIG. 28. When an area of this dark part is large, and the part is visually recognized even in an effective light emitting region of the backlight corresponding to a display screen of the liquid crystal display device, quality level of the backlight largely drops. Especially when the number of LEDs for use is decreased in order to reduce the power consumption, or when a distance between the LED and the effective light emitting region is reduced in order to reduce size of the device, the dark part is easily visually recognized in the effective light emitting region. As a cause of the brightness unevenness, the light emitted from individual LEDs arranged adjacent to the light incident end surface of the light guide has directivity, and further the spread of the light which has entered the light guide becomes comparatively narrow by a refraction function at a time when the light enters the light guide. Furthermore, since the only light going substantially perpendicular to a direction of the elongated prism of the prism sheet is observed in a normal direction of the light outputting surface, the spread of the observed light becomes smaller than that of the light actually output from the light guide. Thus, it has been difficult to establish both the reduction of the power consumption and maintenance of the uniformity of the luminance distribution in the conventional backlight using the point light source as the primary light source.
Further as a method of eliminating the dark part in the vicinity of an incidence surface in the backlight using a linear light source such as a cold cathode tube as the primary light source, for example, in JP(A)-9-160035, a method of roughening the light incident end surface of the light guide has been proposed, but the above-described dark part cannot be sufficiently eliminated in the backlight using the point light source like the LED as the primary light source in this method.
On the other hand, in JP(U)-5-6401 or JP(A)-8-179322, the backlight using the linear light source like the cold cathode tube has been proposed in which a large number of elongated prisms extending in parallel to each other in the direction substantially perpendicular to the light incident end surface are formed on the light outputting surface of the light guide or a back surface thereof for a purpose of converging the output light from the light guide in a plane parallel to the light incident surface. In the light guide in which the elongated prisms are formed, the light introduced into the light guide is directed in a direction in which an inclination angle with respect to the direction of the incident light enlarges or is further returned toward the direction of the incident light by reflection by the elongated prisms of the light guide. Therefore, a travel direction of the light which has entered the light guide is converged in a direction in which the elongated prism extends, and it is therefore possible to enhance luminance. In application of the light guide to the backlight using the LED, the light introduced into the light guide spreads with respect to the direction of the incident light by the reflection by the elongated prism of the light guide, the spread light is output in a direction substantially perpendicular to the elongated prism of the prism sheet, and therefore the distribution of the light seen through the prism sheet appears to spread.
However, when the elongated prism having a cross-sectional shape constituted of a linear portion is formed in the light guide, the light is spread with anisotropy in a specific direction, and therefore bright streaked brightness unevenness is generated in oblique directions as shown in FIG. 29. An actual generation example is shown in FIG. 28. As shown in FIG. 30, generation of the brightness unevenness is seen because the luminance increases in a portion in which the lights output from the respective point light sources are superimposed. An actual generation example is shown in FIG. 31.
Furthermore, when the light incident end surface is roughened as described above in order to eliminate a dark region between the primary light sources or in the corner portion, the dark region is reduced. However, as shown in FIG. 32, bright streaked brightness unevenness is further remarkably observed in oblique directions. An actual generation example is shown in FIG. 33.